4.4-stable review patch. If anyone has any objections, please let me know.
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From: Mario Kleiner mario.kleiner.de@gmail.com
[ Upstream commit 60b95d709525e3ce1c51e1fc93175dcd1755d345 ]
So far we only allowed for 1 retry and just failed the query - and thereby high precision vblank timestamping - if we did not get a reasonable result, as such a failure wasn't considered all too horrible. There are a few NVidia gpu models out there which may need a bit more than 1 retry to get a successful query result under some conditions.
Since Linux 4.4 the update code for vblank counter and timestamp in drm_update_vblank_count() changed so that the implementation assumes that high precision vblank timestamping of a kms driver either consistently succeeds or consistently fails for a given video mode and encoder/connector combo. Iow. switching from success to fail or vice versa on a modeset or connector change is ok, but spurious temporary failure for a given setup can confuse the core code and potentially cause bad miscounting of vblanks and confusion or hangs in userspace clients which rely on vblank stuff, e.g., desktop compositors.
Therefore change the max retry count to a larger number - more than any gpu so far is known to need to succeed, but still low enough so that these queries which do also happen in vblank interrupt are still fast enough to be not disastrously long if something would go badly wrong with them.
As such sporadic retries only happen seldom even on affected gpu's, this could mean a vblank irq could take a few dozen microseconds longer every few hours of uptime -- better than a desktop compositor randomly hanging every couple of hours or days of uptime in a hard to reproduce manner.
Signed-off-by: Mario Kleiner mario.kleiner.de@gmail.com Signed-off-by: Ben Skeggs bskeggs@redhat.com Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/gpu/drm/nouveau/nouveau_display.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/drivers/gpu/drm/nouveau/nouveau_display.c +++ b/drivers/gpu/drm/nouveau/nouveau_display.c @@ -104,7 +104,7 @@ nouveau_display_scanoutpos_head(struct d }; struct nouveau_display *disp = nouveau_display(crtc->dev); struct drm_vblank_crtc *vblank = &crtc->dev->vblank[drm_crtc_index(crtc)]; - int ret, retry = 1; + int ret, retry = 20;
do { ret = nvif_mthd(&disp->disp, 0, &args, sizeof(args));